Heart failure and dilated cardiomyopathy develop in mice that lack the muscle LIM protein (MLP) gene (MLP^−/−). The character and extent of the heart failure that occurs in MLP^−/− mice were investigated using echocardiography and in vivo pressure-volume (P-V) loop measurements. P-V loop data were obtained with a new method for mice (sonomicrometry) using two pairs of orthogonal piezoelectric crystals implanted in the endocardial wall. Sonomicrometry revealed right-shifted P-V loops in MLP^−/−mice, depressed systolic contractility, and additional evidence of heart failure. Cellular changes in MLP^−/− mice were examined in isolated single cells using patch-clamp and confocal Ca²⁺ concentration ([Ca²⁺]) imaging techniques. This cellular investigation revealed unchanged Ca²⁺ currents and Ca²⁺ spark characteristics but decreased intracellular [Ca²⁺] transients and contractile responses and a defect in excitation-contraction coupling. Normal cellular and whole heart function was restored in MLP^−/− mice that express a cardiac-targeted transgene, which blocks the function of β-adrenergic receptor (β-AR) kinase-1 (βARK1). These data suggest that, despite the persistent stimulus to develop heart failure in MLP^−/− mice (i.e., loss of the structural protein MLP), downregulation and desensitization of the β-ARs may play a pivotal role in the pathogenesis. Furthermore, this work suggests that the inhibition of βARK1 action may prove an effective therapy for heart failure.